This disclosure pertains to transmissions and more particularly to two-speed transmissions for marine propulsion.
Shifting from a low speed gear to a high speed gear using a synchronizer normally requires a torque interrupt. In marine applications, this torque interrupt is highly undesirable since it can result in rapid deceleration of the watercraft and an objectionable feel to occupants in the watercraft. For example, a typical shift from a low speed gear to a high speed gear in a marine application may take approximately 0.15 seconds in which time it is common to cut engine speed and reduce the torque load to zero in order to enable shifting from low speed gear and into high gear. This sudden loss of torque leads to the undesirable deceleration and a hard shift since the load on a propeller, especially at cruising speed, is significant.
One known technique to help soften the shift into a high speed gear is to use a one-way overrunning clutch on the output shaft. In such a system, the low speed gear remains engaged to drive the output shaft until the high speed gear is engaged to drive the output shaft at a higher speed than what it is being driven by the low speed gear. An example of such a system is disclosed in U.S. Pat. No. 4,173,939 entitled “Two-Speed Transmission” by Charles Strang, issuing on Nov. 13, 1979. The Strang '939 patent uses a centrifugal clutch to automatically engage the high speed gear in response to rotation of the input shaft above a predetermined speed.
Synchronizer clutch mechanisms are widespread in automotive applications. Traditional synchronizer systems have insufficient torque carrying capacity and thermal capacity for shifting from low speed to high speed in marine applications. Moreover, traditional synchronizers are not capable of locking without a torque interrupt. Despite the apparent drawbacks associated with the use synchronizers for marine applications, it would be desirable to implement synchronizer technology in a way that enables shifting into the high speed gear of a marine transmission without torque interrupt and the associated undesirable deceleration. It would also be desirable to implement such improved synchronizer technology on a vertically oriented marine drive, so that the synchronizer is easy to use in outboard motors and stem drives alike.